The present invention is the fulfillment of a philosophy that would permit users of computer systems (particularly small computer systems whose users would unlikely have resident sophisticated computer programmers) to configure, or tailor, a system to individual needs, or desires, by adding to, or taking from, the system peripheral devices such as video display devices, floppy disc data storage devices, hard disc data storage devices, printers, communication facilities, tape handlers, telephone management systems and the like, without certain constraints and concerns. For instance, in some prior art systems, it has been a user concern that if the user fails to place the physical circuitry, or the correct circuit card, in the correct engagement location of the system chassis, then the peripheral (believed to be connected) would not operate properly and indeed the whole system may not operate properly.
The concept of adding modules to increase the memory capacity of a data processing system, or adding modular peripheral devices to a data processing system to increase the operating features, is well understood in the computer art. However, heretofore a system which permitted such "modular" addition and subtraction required that the system provide a number of hardware (circuitry) features to accommodate the addition of the modules (i.e., interface circuits and peripherals). For instance, with respect to a first feature of prior art systems, each interface circuit card has resident decoding circuitry as part thereof, so that when the master system wants to communicate with a modular device, the interface circuit can be addressed, irrespective of its physical location amongst the slot means which hold such interface circuit (cards). Such decoder circuitry has been fabricated, in the prior art, such that for each rodular device, of a given type (e.g., a floppy disc device), a standard address circuitry configuration is provided, and if the standard address were to necessitate a change, jumpers or wirewrap changes are added, or made, to the address decoding circuitry to "change the address." Thirdly, while such systems do permit a certain amount of interchangeability (because each interface circuit is addressable), such systems have normally required that the interface circuit cards be particularly placed in an ordered arrangement of slots to implement a method for determining priority amongst a group of active interface circuits.
The circuitry which effects a serial scanning of the "ordered" slots has, in the prior art, been some form of "daisy chain" configuration. Such configurations require that there be no empty or vacant slots between active interface circuit cards and this is a constraint which prior art circuits have suffered. In addition, as the number of possible modular peripherals, which could be added, increased, the amount of memory space required for the addresses of the I/O devices, per se, increased. Further since the diagnostic routine instructions, in the prior art, are stored in a memory device of the main system, as the number of modular devices, which optionally could be added, increased, the amount of memory space required to store diagnostic routines for interface circuits (and peripherals per se) increased. The increased use of memory space, described above, of course reduces the amount of memory space available for uses with problem solving programs. Such memory reductions are overcome by adding memory capacity at additional costs.
The present invention eliminates the necessity of having address decoding circuitry on each interface circuit card; eliminates the "ordering" of interface circuit cards in particular slots, to accommodate a priority determination plan; permits vacant slots to exist between active interface circuit cards; eliminates the need to use memory address space to accommodate addresses for increased numbers of optional peripheral devices; and eliminates the waste of memory capacity to accommodate additional diagnostic instructions as the number of optional peripheral devices is increased.